imap-send: use cURL automatically when NO_OPENSSL defined

imap-send: use cURL automatically when NO_OPENSSL defined

If both USE_CURL_FOR_IMAP_SEND and NO_OPENSSL are defined do
not force the user to add --curl to get a working git imap-send
command.

Instead automatically select --curl and warn and ignore the
--no-curl option.  And while we're in there, correct the
warning message when --curl is requested but not supported.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>